Design a concrete pedestal for a steel column carrying a dead load of 600 kN and an imposed load of 600 kN. Assume the base plate is 350 mm x 350 mm M20 grade concrete and F_e, 415 are used.

Factored load = 1.5 (600 + 600) = 1800 kN.

Ans:

Design of Unreinforced Pedestal

Bearing strength = 0.45 f_ck =  0.45 x 20 = 9 N/mm²

Maximum allowable strength = 2 x 9 = 18N/mm².

Pressure on base plate = (1800 x 10³)(350 x 350) = 14.7N/mm²

Area of concrete required = (1800 x 1000)/9 = 200,000 mm²

447 x 447mm.

Provide a pedestal of 450 mm x 450 mm. theoretically the above pedestal need not be reinforced. Usually 0.4 percent of the area of pedestal is provided.

A_s = (0.4 x 450 x 450)/100 = 810 mm²

Provide 4 bars of 16 mm dia.

Area provided = 804.2 m². Provide usual laterals with closer spacing of to 15 cm at the top.

Design of Reinforced Pedestal

Adopt a minimum size for the pedestal, adopting 10 mm clearance, 370mm x 370 mm.

Load carried by the pedestal = 370 x 370 x 9 = 1232kN

Balance load = 1800 - 1232 = 568kN

Area of steel required = 568 x 10³/0.87 x 415 = 1573 mm²

Percatnage of steel = 1573 /(370 x 370) = 1.14%

Provide 4 bars of 20 mm dia.

Area provided = 1576 mm²

Provide usual laterals with closer spacing near the top.